Most air brake systems utilize a sealed housing or brake chamber, an example of which is illustrated schematically in FIG. 1. Within the brake chamber 10 is a diaphragm 12 attached to a push rod 14 that extends through one side 16 of the brake chamber 10. The opposite side of the diaphragm 12 creates an air cavity 18 within the brake chamber 10. The air pressure in the cavity 18 rises when the brake pedal of the vehicle is depressed. The diaphragm 12 deforms, and the air pressure rises, in proportion to the amount of force applied to the brake pedal. The push rod 14 responds to such deformation by moving with the diaphragm 12 to cause engagement of the braking surfaces. When the air brake system is properly adjusted, the push rod 14 travels a distance of up to about two inches from a point at which the brake is completely released to a point at which it is fully actuated. Over time, as the brakes are repeatedly applied, the travel distance of the push rod 14 increases, eventually requiring a readjustment of the brake system before the braking capability deteriorates to a level that renders the vehicle unsafe.
Typical electronic brake stroke sensors currently use magnetic or resistive means to measure the amount of push rod displacement during a braking event. These sensors and electronics are integrated into the brake chamber causing the user to have to purchase and install an entire brake chamber assembly equipped with the stroke sensing equipment. The prior art is generally divided into two categories, namely mechanical visual indicators and electronics integral to the brake chamber assembly. The mechanical visual indicators are not capable of providing the brake stroke information remotely. A person must also be present to observe the brake stroke while another person operates the brake system. Unfavorable weather conditions may discourage visual inspection, which requires crawling under the truck for inspection of each brake stroke indicator. The brake chambers with integrated electronics are expensive and therefore not good candidates for retrofit applications. Additionally, the installation is a much more complex process than would be acceptable for a retrofit into existing equipment.
Accordingly, there is a present need for an electronic brake stroke sensor capable of integration into an existing brake system without the need for extensive redesign of the brake chamber. Moreover, there is a present need for an electronic brake stroke sensor that requires no moving parts and can be installed without regard to alignment and provides for self-calibration, thus avoiding the problem of misalignment or inaccurate readings.